Clinical Dilemma: The frequency of impaired fracture healing is increased with aging as well as in the presence of other patient-related factors such as smoking, osteoporosis, and diabetes. Treatment of fractures in this setting continues to pose a significant economic burden on the US healthcare system due to increases in time lost from work as well as increases in the expenses associated with fracture-associated complications. While various bone anabolic drugs are successful in increasing homeostatic bone mass in osteoporotic patients and decreasing fracture incidence, they have not demonstrated significant success in enhancing fracture repair. Therefore, identifying novel molecular targets to accelerate secondary fracture healing in this very common setting remains of paramount importance. Relevance to the VA: According to the Office of VA Inspector General report in 2010, osteoporotic patients who suffered a single fracture present a higher incidence of subsequent fractures (20-fold increase) than unaffected populations. Impaired or delayed bony union following fracture of long bones prevents or delays a significant percentage of VA patients from resuming their daily activities and returning to work. Ineffective treatment of these fractures maximizes the economic burden on the VA healthcare system. Identifying novel molecular targets to enhance secondary bone repair remains of paramount importance. The objective of this translational research application is to enhance secondary fracture healing by targeting novel regulatory pathways that enhance periosteal cell-induced osteogenesis and angiogenesis during fracture callus formation. Scientific premise: We provide compelling preliminary evidence of the following: 1. Runx3 is expressed in chondrocytes, osteoblasts and osteocytes of C57BL6j murine long bones. 2. Runx3 expression levels are increased in soft cartilaginous calluses and subsequently decreased in bony calluses of murine femoral fractures. 3. Conditional deletion of Runx3 in periosteal cells (cKO) resulted in enhanced secondary bone healing as evidenced by histological, histomorphometric, and molecular analyses. 4. The cellular mechanisms underlying these positive effects on secondary bone healing implicate increased osteogenesis as well as angiogenesis of fractured periosteal cells from Runx3 cKO compared to control mice. 5. Use of multiphoton microscopy demonstrate the feasibility of tracking Prx1+ skeletal progenitor cells during bone repair and longitudinally monitor the bone healing process for lineage tracing experiments. Here we hypothesize that Runx3 is a molecular switch that controls the transition from cartilaginous to bony callus, and its deletion in the chondrogenic cell lineage will accelerate secondary fracture healing. To verify this hypothesis, we propose to first establish the effects of stage-specific repression of Runx3 on secondary fracture healing (Aim 1). We will then determine the mechanisms via which Runx3 controls mesenchymal cell differentiation into the chondro/osteogenic lineages (Aim 2). Finally, we will assess the efficacy of Runx3 inhibition during fracture repair in control C57BL6 mice through controlled and sustained delivery of miRNA encapsulated hydrogel and examine the rate of bone healing and biomechanical strength of healed bone (Aim 3). Impact: Defining the pathways that governthe transition from soft to bony callus will help identify new therapies to accelerate secondary fracture healing. Here, we will establish Runx3 as a novel therapeutic target.